Method and apparatus for treating irregular ventricular contractions such as during atrial arrhythmia

ABSTRACT

A cardiac rhythm management system is capable of treating irregular ventricular heart contractions, such as during atrial tachyarrhythmias such as atrial fibrillation. A first indicated pacing interval is computed based at least partially on a most recent V-V interval duration between ventricular beats and a previous value of the first indicated pacing interval. Pacing therapy is provided based on either the first indicated pacing interval or also based on a second indicated pacing interval, such as a sensor-indicated pacing interval. A weighted averager such as an infinite impulse response (IIR) filter adjusts the first indicated pacing interval for sensed beats and differently adjusts the first indicated pacing interval for paced beats. The system regularizes ventricular rhythms by pacing the ventricle, but inhibits pacing when the ventricular rhythms are stable.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. patent applicationSer. No. 09/316,515, filed on May 21, 1999, the specification of whichis incorporated herein by reference in its entirety.

[0002] This application is related to the following, commonly assignedpatent applications: “Cardiac Rhythm Management System Promoting AtrialPacing,” U.S. Pat. No. 6,351,669; “Cardiac Rhythm Management System WithAtrial Shock Timing Optimization,” U.S. Pat. No. 6,430,438; and “SystemProviding Ventricular Pacing and Biventricular Coordination,” U.S. Pat.No. 6,285,907; each of which were filed on May 21, 1999, each of whichdisclosure is herein incorporated by reference in its entirety.

TECHNICAL FIELD

[0003] The present system relates generally to cardiac rhythm managementsystems and particularly, but not by way of limitation, to a method andapparatus for treating irregular ventricular contractions, such asduring an atrial arrhythmia.

BACKGROUND

[0004] When functioning properly, the human heart maintains its ownintrinsic rhythm, and is capable of pumping adequate blood throughoutthe body's circulatory system. However, some people have irregularcardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmiasresult in diminished blood circulation. One mode of treating cardiacarrhythmias uses drug therapy. Drugs are often effective at restoringnormal heart rhythms. However, drug therapy is not always effective fortreating arrhythmias of certain patients. For such patients, analternative mode of treatment is needed. One such alternative mode oftreatment includes the use of a cardiac rhythm management system. Suchsystems are often implanted in the patient and deliver therapy to theheart.

[0005] Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacers deliver timed sequencesof low energy electrical stimuli, called pace pulses, to the heart, suchas via an intravascular leadwire or catheter (referred to as a “lead”)having one or more electrodes disposed in or about the heart. Heartcontractions are initiated in response to such pace pulses (this isreferred to as “capturing” the heart). By properly timing the deliveryof pace pulses, the heart can be induced to contract in proper rhythm,greatly improving its efficiency as a pump. Pacers are often used totreat patients with bradyarrhythmias, that is, hearts that beat tooslowly, or irregularly.

[0006] Cardiac rhythm management systems also include cardioverters ordefibrillators that are capable of delivering higher energy electricalstimuli to the heart. Defibrillators are often used to treat patientswith tachyarrhythmias, that is, hearts that beat too quickly. Suchtoo-fast heart rhythms also cause diminished blood circulation becausethe heart isn't allowed sufficient time to fill with blood beforecontracting to expel the blood. Such pumping by the heart isinefficient. A defibrillator is capable of delivering an high energyelectrical stimulus that is sometimes referred to as a defibrillationcountershock. The countershock interrupts the tachyarrhythmia, allowingthe heart to reestablish a normal rhythm for the efficient pumping ofblood. In addition to pacers, cardiac rhythm management systems alsoinclude, among other things, pacer/defibrillators that combine thefunctions of pacers and defibrillators, drug delivery devices, and anyother implantable or external systems or devices for diagnosing ortreating cardiac arrhythmias.

[0007] One problem faced by cardiac rhythm management systems is theproper treatment of ventricular arrhythmias that are caused by atrialtachyarrhythmias such as atrial fibrillation. Atrial fibrillation is acommon cardiac arrhythmia which reduces the pumping efficiency of theheart, though not to as great a degree as in ventricular fibrillation.However, this reduced pumping efficiency requires the ventricle to workharder, which is particularly undesirable in sick patients that cannottolerate additional stresses. As a result of atrial fibrillation,patients may be required to limit their activity and exercise.

[0008] Although atrial fibrillation, by itself, is usually notlife-threatening, prolonged atrial fibrillation may be associated withstrokes, which are thought to be caused by blood clots forming in areasof stagnant blood flow. Treating such blood clots requires the use ofanticoagulants. Atrial fibrillation may also cause pain, dizziness, andother irritation to the patient.

[0009] An even more serious problem, however, is that atrialfibrillation may induce irregular ventricular heart rhythms by processesthat are yet to be fully understood. Such induced ventriculararrhythmias compromise pumping efficiency even more drastically thanatrial arrhythmias. For these and other reasons, there is a need for amethod and apparatus for treating irregular ventricular contractionsduring atrial arrhythmias such as atrial fibrillation.

SUMMARY OF THE INVENTION

[0010] The present system provides a method and apparatus for treatingirregular ventricular contractions, such as during atrial arrhythmias(e.g., atrial fibrillation), or otherwise. The present system providesmany advantages. Among other things, it is capable of treating irregularventricular heart contractions, such as during atrial tachyarrhythmias.It provides a first indicated pacing rate that increases for sensedventricular beats and decreases for paced ventricular beats. The systemdelivers more pacing during irregular sensed beats (such as duringatrial tachyarrhythmias including atrial fibrillation or the like) andless pacing when sensed beats are regular. In a stable heart, the firstindicated pacing rate is typically less than the intrinsic heart rate.This avoids unnecessary pacing of the heart when heart rhythms aresubstantially stable, allowing the heart to beat normally and at its ownintrinsic heart rate.

[0011] One aspect of the system permits it to avoid rapid changes inheart rate, and to keep heart rate within acceptable upper and lowerlimits. The system allows the rate to become more regular and toapproach a stable rhythm. This provides improved comfort of the patientexperiencing irregular ventricular contractions. In a furtherembodiment, the system includes a second indicated pacing rate, such asa sensor-indicated rate, and provides pacing therapy based on both thefirst and second indicated pacing rates. Other aspects of the inventionwill be apparent on reading the following detailed description of theinvention and viewing the drawings that form a part thereof.

[0012] In one embodiment, the system obtains V-V intervals betweenventricular beats. A first indicated pacing interval is computed basedat least partially on a most recent V-V interval duration and a previousvalue of the first indicated pacing interval. Pacing therapy is providedbased on the first indicated pacing interval.

[0013] In a further embodiment, the first indicated pacing interval isadjusted by an amount based at least on the most recent V-V intervalduration and the previous value of the first indicated pacing interval,if the most recent V-V interval is concluded by an intrinsic beat. If,however, the most recent V-V interval is concluded by a paced beat, thenthe first indicated pacing interval is increased by an amount based atleast on the most recent V-V interval duration and the previous value ofthe first indicated pacing interval.

[0014] In another embodiment, the system detects an atrialtachyarrhythmia. The system obtains V-V intervals between ventricularbeats. A first indicated pacing interval is computed based at leastpartially on a most recent V-V interval duration and a previous value ofthe first indicated pacing interval. Pacing therapy is provided based onthe first indicated pacing interval, if the atrial tachyarrhythmia ispresent.

[0015] One embodiment provides a cardiac rhythm management system thatincludes, among other things, a ventricular sensing circuit, acontroller, and a ventricular therapy circuit. The controller includes aV-V interval timer, a first register, for storing information associatedwith a first indicated pacing interval, and a filter that updates thefirst indicated pacing interval based on the V-V interval timer and theinformation stored in the first register. Other aspects of the inventionwill be apparent on reading the following detailed description of theinvention and viewing the drawings that form a part thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the drawings, like numerals describe substantially similarcomponents throughout the several views. Like numerals having differentletter suffixes represent different instances of substantially similarcomponents.

[0017]FIG. 1 is a schematic drawing illustrating generally oneembodiment of portions of a cardiac rhythm management system and anenvironment in which it is used.

[0018]FIG. 2 is a schematic drawing illustrating one embodiment of acardiac rhythm management device coupled by leads to a heart.

[0019]FIG. 3 is a schematic diagram illustrating generally oneembodiment of portions of a cardiac rhythm management device coupled toa heart.

[0020]FIG. 4 is a schematic diagram illustrating generally oneembodiment of a controller.

[0021]FIG. 5 is a schematic diagram illustrating generally oneconceptualization of portions of a controller.

[0022]FIG. 6 is a signal flow diagram illustrating generally oneconceptual embodiment of operating a filter.

[0023]FIG. 7 is a signal flow diagram illustrating generally anotherconceptualization of operating the filter.

[0024]FIG. 8 is a signal flow diagram illustrating generally a furtherconceptualization of operating the filter.

[0025]FIG. 9 is a schematic diagram illustrating generally anotherconceptualization of portions of a controller.

[0026]FIG. 10 is a schematic diagram illustrating generally a furtherconceptualization of portions of a controller.

[0027]FIG. 11 is a graph illustrating generally one embodiment ofoperating a filter to provide a first indicated pacing rate, such as aVRR indicated rate, for successive ventricular heart beats.

[0028]FIG. 12 is a graph illustrating generally another embodiment ofoperating a filter to provide the first indicated pacing rate, such as aVRR indicated rate, and delivering therapy based on the first indicatedpacing rate and based on a second indicated pacing rate, such as asensor indicated rate.

[0029]FIG. 13 is a graph illustrating generally another illustrativeexample of heart rate vs. time according to a VRR algorithm spreadsheetsimulation.

[0030]FIG. 14 is a graph illustrating generally one embodiment of usingat least one of coefficients a and b as a function of heart rate (or acorresponding time interval).

DETAILED DESCRIPTION

[0031] In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims and their equivalents. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. Like numerals having different lettersuffixes represent different instances of substantially similarcomponents.

[0032] The present methods and apparatus will be described inapplications involving implantable medical devices including, but notlimited to, implantable cardiac rhythm management systems such aspacemakers, cardioverter/defibrillators, and pacer/defibrillators.However, it is understood that the present methods and apparatus may beemployed in unimplanted devices, including, but not limited to, externalpacemakers, cardioverter/defibrillators, pacer/defibrillators, monitors,programmers and recorders.

Problems Associated With Atrial Arrhythmias

[0033] As stated earlier, one potential cause of irregularity ofventricular contractions arises during atrial tachyarrhythmias, such asatrial fibrillation. During atrial fibrillation, irregular ventricularcontractions may be caused by a conducted atrial tachyarrhythmia, thenpacing the ventricle will regularize the ventricular heart rate byestablishing retrograde conduction from the ventricle. This, in turn, isbelieved to block forward conduction of atrial signals through theatrioventricular (A-V) node. As a result, irregular atrial signals donot trigger resulting irregular ventricular contractions.

[0034] One therapy for treating irregular ventricular contractionsduring atrial fibrillation is to increase the ventricular heart rate bypacing the ventricle at a higher rate than the unpaced (intrinsic)ventricular heart rate. Such therapy is believed to decrease thediscomfort experienced by the patient having atrial arrhythmia becauseit regulates the ventricular contractions to avoid short periods betweencontractions and/or long periods without a contraction. Such therapy isalso believed to decrease the ability of the atrial fibrillation toinduce irregular ventricular contractions.

[0035] An increase in rate of ventricular contractions, however, must bedone carefully to avoid pacing the heart at an unnecessarily high rate.Furthermore, such a therapy should not impose pacing where normal or“intrinsic” heart pacing is adequate such as, for example, when atrialtachyarrhythmias no longer cause disorder of ventricular contractions.As long as the heart is actively paced, it may be difficult orimpossible to determine when to cease such a therapy. One advantage ofthe present system is that it allows intrinsic ventricular rhythms, ifsuch rhythms are regular, but provides pacing that stabilizesventricular rhythms if they become irregular, as discussed below.

General System Overview and Examples

[0036] This document describes, among other things, a cardiac rhythmmanagement system providing a method and apparatus for treatingirregular ventricular contractions during atrial arrhythmia. FIG. 1 is aschematic drawing illustrating, by way of example, but not by way oflimitation, one embodiment of portions of a cardiac rhythm managementsystem 100 and an environment in which it is used. In FIG. 1, system 100includes an implantable cardiac rhythm management device 105, alsoreferred to as an electronics unit, which is coupled by an intravascularendocardial lead 110, or other lead, to a heart 115 of patient 120.System 100 also includes an external programmer 125 providing wirelesscommunication with device 105 using a telemetry device 130. Catheterlead 110 includes a proximal end 135, which is coupled to device 105,and a distal end 140, which is coupled to one or more portions of heart115.

[0037]FIG. 2 is a schematic drawing illustrating, by way of example, butnot by way of limitation, one embodiment of device 105 coupled by leads110A-B to heart 115, which includes a right atrium 200A, a left atrium200B, a right ventricle 205A, a left ventricle 205B, and a coronarysinus 220 extending from right atrium 200A. In this embodiment, atriallead 110A includes electrodes (electrical contacts) disposed in, around,or near an atrium 200 of heart 115, such as ring electrode 225 and tipelectrode 230, for sensing signals and/or delivering pacing therapy tothe atrium 200. Lead 110A optionally also includes additionalelectrodes, such as for delivering atrial and/or ventricularcardioversion/defibrillation and/or pacing therapy to heart 115.

[0038] In FIG. 2, a ventricular lead 110B includes one or moreelectrodes, such as tip electrode 235 and ring electrode 240, fordelivering sensing signals and/or delivering pacing therapy. Lead 110Boptionally also includes additional electrodes, such as for deliveringatrial and/or ventricular cardioversion/defibrillation and/or pacingtherapy to heart 115. Device 105 includes components that are enclosedin a hermetically-sealed can 250. Additional electrodes may be locatedon the can 250, or on an insulating header 255, or on other portions ofdevice 105, for providing unipolar pacing and/or defibrillation energyin conjunction with the electrodes disposed on or around heart 115.Other forms of electrodes include meshes and patches which may beapplied to portions of heart 115 or which may be implanted in otherareas of the body to help “steer” electrical currents produced by device105. The present method and apparatus will work in a variety ofconfigurations and with a variety of electrical contacts or“electrodes.”

Example Cardiac Rhythm Management Device

[0039]FIG. 3 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of portions ofdevice 105, which is coupled to heart 115. Device 105 includes a powersource 300, an atrial sensing circuit 305, a ventricular sensing circuit310, a ventricular therapy circuit 320, and a controller 325.

[0040] Atrial sensing circuit 305 is coupled by atrial lead 110A toheart 115 for receiving, sensing, and/or detecting electrical atrialheart signals. Such atrial heart signals include atrial activations(also referred to as atrial depolarizations or P-waves), whichcorrespond to atrial contractions. Such atrial heart signals includenormal atrial rhythms, and abnormal atrial rhythms including atrialtachyarrhythmias, such as atrial fibrillation, and other atrialactivity. Atrial sensing circuit 305 provides one or more signals tocontroller 325, via node/bus 327, based on the received atrial heartsignals. Such signals provided to controller 325 indicate, among otherthings, the presence of atrial fibrillation.

[0041] Ventricular sensing circuit 310 is coupled by ventricular lead10B to heart 115 for receiving, sensing, and/or detecting electricalventricular heart signals, such as ventricular activations (alsoreferred to as ventricular depolarizations or R-waves), which correspondto ventricular contractions. Such ventricular heart signals includenormal ventricular rhythms, and abnormal ventricular rhythms, includingventricular tachyarrhythmias, such as ventricular fibrillation, andother ventricular activity, such as irregular ventricular contractionsresulting from conducted signals from atrial fibrillation. Ventricularsensing circuit 310 provides one or more signals to controller 325, vianode/bus 327, based on the received ventricular heart signals. Suchsignals provided to controller 325 indicate, among other things, thepresence of ventricular depolarizations, whether regular or irregular inrhythm.

[0042] Ventricular therapy circuit 320 provides ventricular pacingtherapy, as appropriate, to electrodes located at or near one of theventricles 205 of heart 115 for obtaining resulting evoked ventriculardepolarizations. In one embodiment, ventricular therapy circuit 320 alsoprovides cardioversion/defibrillation therapy, as appropriate, toelectrodes located at or near one of the ventricles 205 of heart 115,for terminating ventricular fibrillation and/or other ventriculartachyarrhythmias.

[0043] Controller 325 controls the delivery of therapy by ventriculartherapy circuit 320 and/or other circuits, based on heart activitysignals received from atrial sensing circuit 305 and ventricular sensingcircuit 310, as discussed below. Controller 325 includes variousmodules, which are implemented either in hardware or as one or moresequences of steps carried out on a microprocessor or other controller.Such modules are illustrated separately for conceptual clarity; it isunderstood that the various modules of controller 325 need not beseparately embodied, but may be combined and/or otherwise implemented,such as in software/firmware.

[0044] In general terms, sensing circuits 305 and 310 sense electricalsignals from heart tissue in contact with the catheter leads 11A-B towhich these sensing circuits 305 and 310 are coupled. Sensing circuits305 and 310 and/or controller 325 process these sensed signals. Based onthese sensed signals, controller 325 issues control signals to therapycircuits, such as ventricular therapy circuit 320, if necessary, for thedelivery of electrical energy (e.g., pacing and/or defibrillationpulses) to the appropriate electrodes of leads 110A-B. Controller 325may include a microprocessor or other controller for execution ofsoftware and/or firmware instructions. The software of controller 325may be modified (e.g., by remote external programmer 105) to providedifferent parameters, modes, and/or functions for the implantable device105 or to adapt or improve performance of device 105.

[0045] In one further embodiment, one or more sensors, such as sensor330, may serve as inputs to controller 325 for adjusting the rate atwhich pacing or other therapy is delivered to heart 115. One such sensor330 includes an accelerometer that provides an input to controller 325indicating increases and decreases in physical activity, for whichcontroller 325 increases and decreases pacing rate, respectively.Another such sensor includes an impedance measurement, obtained frombody electrodes, which provides an indication of increases and decreasesin the patient's respiration, for example, for which controller 325increases and decreases pacing rate, respectively. Any other sensor 330providing an indicated pacing rate can be used.

[0046]FIG. 4 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of controller 325that includes several different inputs to modify the rate at whichpacing or other therapy is delivered. For example, Input #1 may provideinformation about left ventricular rate, Input #2 may provide anaccelerometer-based indication of activity, and Input #3 may provide animpedance-based indication of respiration, such as minute ventilation.Based on at least one of these and/or other inputs, controller 325provides an output indication of pacing rate as a control signaldelivered to a therapy circuit, such as to ventricular therapy circuit320. Ventricular therapy circuit 320 issues pacing pulses based on oneor more such control signals received from controller 325. Control ofthe pacing rate may be performed by controller 325, either alone or incombination with peripheral circuits or modules, using software,hardware, firmware, or any combination of the like. The softwareembodiments provide flexibility in how inputs are processed and may alsoprovide the opportunity to remotely upgrade the device software whilestill implanted in the patient without having to perform surgery toremove and/or replace the device 105.

Controller Example 1

[0047]FIG. 5 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, one conceptualization of portionsof controller 325. At least one signal from ventricular sensing circuit310 is received by ventricular event module 500, which recognizes theoccurrence of ventricular events included within the signal. Such eventsare also referred to as “beats,” “activations,” “depolarizations,” “QRScomplexes,” “R-waves,” “contractions.” Ventricular event module 500detects intrinsic events (also referred to as sensed events) from thesignal obtained from ventricular sensing circuit 310. Ventricular eventmodule 500 also detects evoked events (resulting from a pace) eitherfrom the signal obtained from ventricular sensing circuit 310, orpreferably from a ventricular pacing control signal obtained from pacingcontrol module 505, which also triggers the delivery of a pacingstimulus by ventricular therapy circuit 320. Thus, ventricular eventsinclude both intrinsic/sensed events and evoked/paced events.

[0048] A time interval between successive ventricular events, referredto as a V-V interval, is recorded by a first timer, such as V-V intervaltimer 510. A filter 515 computes a “first indicated pacing interval,”i.e., one indication of a desired time interval between ventricularevents or, stated differently, a desired ventricular heart rate. Thefirst indicated pacing interval is also referred to as a ventricularrate regularization (VRR) indicated pacing interval. In variousembodiments, filter 515 includes an averager, a weighted averager, amedian filter, an infinite (IIR) filter, a finite impulse response (FIR)filter, or any other analog or digital signal processing circuitproviding the desired signal processing described more particularlybelow.

[0049] In one embodiment, filter 515 computes a new value of the firstindicated pacing interval based on the duration of the most recent V-Vinterval recorded by timer 510 and on a previous value of the firstindicated pacing interval stored in first indicated pacing intervalregister 520. Register 520 is then updated by storing the newly computedfirst indicated pacing interval in register 520. Based on the firstindicated pacing interval stored in register 520, pacing control module505 delivers control signals to ventricular therapy circuit 320 fordelivering therapy, such as pacing stimuli, at the VRR-indicatedventricular heart rate corresponding to the inverse of the duration ofthe first indicated pacing interval.

Filter Example 1

[0050] In general terms, for one embodiment, device 105 obtains V-Vintervals between successive sensed or evoked ventricular beats. Device105 computes a new first indicated pacing interval based at least inpart on the duration of the most recent V-V interval and a previousvalue of the first indicated pacing interval. Device 105 provides pacingtherapy delivered at a rate corresponding to the inverse of the durationof the first indicated pacing interval.

[0051]FIG. 6 is a signal flow diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of operatingfilter 515. Upon the occurrence of a sensed or evoked ventricular beat,timer 510 provides filter 515 with the duration of the V-V intervalconcluded by that beat, which is referred to as the most recent V-Vinterval (VV_(n)). Filter 515 also receives the previous value of thefirst indicated pacing interval (T_(n-1)) stored in register 520. Themost recent V-V interval VV_(n) and the previous value of the firstindicated pacing interval T_(n-1) are each scaled by respectiveconstants A and B, and then summed to obtain a new value of the firstindicated pacing interval (T_(n)), which is stored in register 520 andprovided to pacing control module 505. In one embodiment, thecoefficients A and B are different values, and are either programmable,variable, or constant.

[0052] If no ventricular beat is sensed during the new first indicatedpacing interval T_(n), which is measured as the time from the occurrenceof the ventricular beat concluding the most recent V-V interval VV_(n),then pacing control module 505 instructs ventricular therapy circuit 320to deliver a ventricular pacing pulse upon the expiration of the newfirst indicated pacing interval T_(n). In one embodiment, operation ofthe filter is described by T_(n)=A·VV_(n)+B·T_(n-1), where A and B arecoefficients (also referred to as “weights”), VV_(n) is the most recentV-V interval duration, and T_(n-1) is the previous value of the firstindicated pacing interval.

[0053] Initialization of filter 515 includes seeding the filter bystoring, in register 520, an initial interval value. In one embodiment,register 520 is initialized to an interval value corresponding to alower rate limit (LRL), i.e., a minimum rate at which pacing pulses aredelivered by device 105. Register 520 could alternatively be initializedwith any other suitable value.

Filter Example 2

[0054] In one embodiment, operation of filter 515 is based on whetherthe beat concluding the most recent V-V interval VV_(n) is asensed/intrinsic beat or a paced/evoked beat. In this embodiment, thepacing control module 505, which controls the timing and delivery ofpacing pulses, provides an input to filter 515 that indicates whetherthe most recent V-V interval VV_(n) was concluded by an evoked beatinitiated by a pacing stimulus delivered by device 105, or was concludedby an intrinsic beat sensed by ventricular sensing circuit 310.

[0055] In general terms, if the most recent V-V interval VV_(n) isconcluded by a sensed/intrinsic beat, then filter 515 provides a newfirst indicated pacing interval T_(n) that is adjusted from the value ofthe previous first indicated pacing interval T_(n-1) such as, forexample, decreased by an amount that is based at least partially on theduration of the most recent V-V interval VV_(n) and on the duration ofthe previous value of the first indicated pacing interval T_(n-1). If,however, the most recent V-V interval VVn is concluded by a paced/evokedbeat, then filter 515 provides a new first indicated pacing intervalT_(n) that is increased from the value of the previous first indicatedpacing interval T_(n-1), such as, for example, by an amount that isbased at least partially on the duration of the most recent V-V intervalVV_(n) and on the duration of the previous value of the first indicatedpacing interval T_(n-1). If no ventricular beat is sensed during the newfirst indicated pacing interval T_(n), which is measured as the timefrom the occurrence of the ventricular beat concluding the most recentV-V interval VV_(n), then pacing control module 505 instructsventricular therapy circuit 320 to deliver a ventricular pacing pulseupon the expiration of the new first indicated pacing interval T_(n).

[0056]FIG. 7 is a signal flow diagram, illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofoperating filter 515, with certain differences from FIG. 6 moreparticularly described below. In this embodiment, the pacing controlmodule 505, which controls the timing and delivery of pacing pulses,provides an input to filter 515 that indicates whether the most recentV-V interval VV_(n) was concluded by an evoked beat initiated by apacing stimulus delivered by device 105, or was concluded by anintrinsic beat sensed by ventricular sensing circuit 310.

[0057] If the most recent V-V interval VV_(n) was concluded by anintrinsic beat, then the most recent V-V interval VV_(n) and theprevious value of the first indicated pacing interval T_(n-1) are eachscaled by respective constants A and B, and then summed to obtain thenew value of the first indicated pacing interval T_(n), which is storedin register 520 and provided to pacing control module 505.Alternatively, if the most recent V-V interval VV_(n) was concluded by aevoked/paced beat, then the most recent V-V interval VV_(n) and theprevious value of the first indicated pacing interval T_(n-1) are eachscaled by respective constants C and D, and then summed to obtain thenew value of the first indicated pacing interval T_(n), which is storedin register 520 and provided to pacing control module 505. In oneembodiment, the coefficients C and D are different from each other, andare either programmable, variable, or constant. In a further embodiment,the coefficient C is a different value from the coefficient A, and/orthe coefficient D is a different value than the coefficient B, and thesecoefficients are either programmable, variable, or constant. In anotherembodiment, the coefficient D is the same value as the coefficient B.

[0058] In one embodiment, operation of filter 515 is described byT_(n)=A·VV_(n)+B·T_(n-1), if VV_(n) is concluded by an intrinsic beat,and is described by T_(n)=C·VV_(n)+D·T_(n-1), if VV_(n) is concluded bya paced beat, where A, B, C and D are coefficients (also referred to as“weights”), VV_(n) is the most recent V-V interval duration, T_(n) isthe new value of the first indicated pacing interval, and T_(n-1) is theprevious value of the first indicated pacing interval. If no ventricularbeat is sensed during the new first indicated pacing interval T_(n),which is measured as the time from the occurrence of the ventricularbeat concluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the new first indicatedpacing interval T_(n).

Filter Example 3

[0059] In another embodiment, these coefficients can be moreparticularly described using an intrinsic coefficient (a), a pacedcoefficient (b), and a weighting coefficient (w). In one suchembodiment, A=a·w, B=(1−w), C=b·w, and D=(1−w). In one example,operation of the filter 515 is described byT_(n)=a·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by an intrinsicbeat, otherwise is described by T_(n)=b·w·VV_(n)+(1−w)·T_(n-1), ifVV_(n) is concluded by a paced beat, as illustrated generally, by way ofexample, but not by way of limitation, in the signal flow graph of FIG.8. If no ventricular beat is sensed during the new first indicatedpacing interval T_(n), which is measured as the time from the occurrenceof the ventricular beat concluding the most recent V-V interval VV_(n),then pacing control module 505 instructs ventricular therapy circuit 320to deliver a ventricular pacing pulse upon the expiration of the newfirst indicated pacing interval T_(n). In one embodiment, thecoefficients a and b are different from each other, and are eitherprogrammable, variable, or constant.

[0060] The above-described parameters (e.g., A, B, C, D, a, b, w) arestated in terms of time intervals (e.g., VV_(n), T_(n), T_(n-1)).However, an alternate system may produce results in terms of rate,rather than time intervals, without departing from the present methodand apparatus. In one embodiment, weighting coefficient w, intrinsiccoefficient a, and paced coefficient b, are variables. Differentselections of w, a, and b, will result in different operation of thepresent method and apparatus. For example, as w increases the weightingeffect of the most recent V-V interval VV_(n) increases and theweighting effect of the previous first indicated pacing rate T_(n-1)decreases. In one embodiment, w ={fraction (1/16)}=0.0625. In anotherembodiment, w={fraction (1/32)}. Another possible range for w is fromw=½ to w={fraction (1/1024)}. A further possible range for w is from w≈0to w≈1. Other values of w, which need not include division by powers oftwo, may be substituted without departing from the present method andapparatus.

[0061] In one embodiment, intrinsic coefficient a, is selected to begreater than 0.5, or to be greater than 1.0. In one example, theintrinsic coefficient a is selected to be lesser in value than thepacing coefficient b. In one example, a≈1.1 and b≈1.2. In anotherembodiment a=0.9 and b=1.1. One possible range for a is from a=0.5 toa=2.0, and for b is from b=1.0 to b=3.0. The coefficients may varywithout departing from the present method and apparatus.

[0062] In one embodiment, for b>1 and for substantially regular V-Vintervals, filter 515 provides a new first indicated pacing intervalT_(n) that is at least slightly longer than the expected intrinsic V-Vinterval being measured by timer 515. Thus, if the intrinsic V-Vinterval being timed is consistent with the duration of previouslyreceived V-V intervals, then filter 515 avoids triggering a pacingstimulus. In such a case, a pacing pulse is delivered only if thepresently timed V-V interval becomes longer than the previoussubstantially constant V-V intervals. In general terms, filter 515operates so that pacing pulses are typically inhibited if theventricular rate is substantially constant. However, if the measured V-Vintervals become irregular, then filter 515 operates, over a period ofone or several such V-V intervals, to shorten the first indicated pacinginterval T_(n) so that pacing stimuli are being delivered.

[0063] According to one aspect of the invention, it is believed that ifthe irregular V-V intervals are caused by a conducted atrialtachyarrhythmia, then pacing the ventricle will regularize theventricular heart rate by establishing retrograde conduction from theventricle. This, in turn, blocks forward conduction of atrial signalsthrough the atrioventricular (A-V) node. As a result, irregular atrialsignals do not trigger resulting irregular ventricular contractions.According to another aspect of the invention, however, this method andapparatus will not introduce pacing pulses until the heartbeat becomesirregular. Therefore, the heart is assured to pace at its intrinsic ratewhen regular ventricular contractions are sensed.

Controller Example 2

[0064]FIG. 9 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofportions of controller 325, with certain differences from FIG. 5 moreparticularly described below. In FIG. 9, controller 325 receives fromsensor 330 a signal including information from which a physiologicallydesired heart rate (e.g., based on the patient's activity, respiration,or any other suitable indicator of metabolic need) can be derived. Thesensor signal is digitized by an A/D converter 900. The digitized signalis processed by a sensor rate module 905, which computes a desired heartrate that is expressed in terms of a second indicated pacing intervalstored in register 910.

[0065] Pacing control module 505 delivers a control signal, whichdirects ventricular therapy circuit 320 to deliver a pacing pulse, basedon either (or both) of the first or second indicated pacing intervals,stored in registers 520 and 910, respectively, or both. In oneembodiment, pacing control module 505 includes a selection module 915that selects between the new first indicated pacing interval T_(n) andthe sensor-based second indicated pacing interval.

[0066] In one embodiment, selection module 915 selects the shorter ofthe first and second indicated pacing intervals as the selectedindicated pacing interval S_(n). If no ventricular beat is sensed duringthe selected indicated pacing interval S_(n), which is measured as thetime from the occurrence of the ventricular beat concluding the mostrecent V-V interval VV_(n), then pacing control module 505 instructsventricular therapy circuit 320 to deliver a ventricular pacing pulseupon the expiration of the selected indicated pacing interval S_(n).

[0067] In general terms, for this embodiment, the ventricle is paced atthe higher of the sensor indicated rate and the VRR indicated rate. If,for example, the patient is resting, such that the sensor indicated rateis lower than the patient's intrinsic rate, and the patient's intrinsicrate is substantially constant, then the intrinsic rate is higher thanthe VRR indicated rate. As a result, pacing pulses generally will not bedelivered. But if, for example, the patient is resting, but with anatrial tachyarrhythmia that induces irregular ventricular contractions,then pacing pulses generally will be delivered at the VRR indicatedrate. In another example, if the patient is active, such that the sensorindicated rate is higher than the VRR indicated rate, then pacing pulsesgenerally will be delivered at the sensor indicated rate. In analternative embodiment, the pacing rate is determined by blending thesensor indicated rate and the VRR indicated rate, rather than byselecting the higher of these two indicated rates (i.e., the shorter ofthe first and second indicated pacing intervals).

[0068] In another embodiment, selection module 915 provides a selectedindicated pacing interval S_(n) based on a blending of both the firstand second indicated pacing intervals. In one such example, selectionmodule 915 applies predetermined or other weights to the first andsecond indicated pacing intervals to compute the selected pacinginterval S_(n).

Controller Example 2

[0069]FIG. 10 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofportions of controller 325, with certain differences from FIG. 9 moreparticularly described below. In FIG. 10, controller 325 includes anatrial tachyarrhythmia (AT) detection module 1000 that receives a signalfrom atrial sensing circuit 305. The received signal includesinformation about atrial events, from which AT detection module 1000determines the presence or absence of one or more atrialtachyarrhythmias, such as atrial fibrillation.

[0070] In one embodiment, AT detection module 1000 provides a controlsignal, to pacing control module 505, that indicates the presence orabsence of an atrial tachyarrhythmia, such as atrial fibrillation. Inone embodiment, selection module 915 selects between the first andsecond indicated pacing intervals as illustrated, by way of example, butnot by way of limitation, in Table 1. TABLE 1 Example Selection Based onAT Detection, 1st Indicated Pacing Interval, and 2nd Indicated PacingInterval 1st Indicated Pacing 1st Indicated Pacing Interval < 2ndIndicated Interval ≧ 2nd Indicated AT Present? Pacing Interval? PacingInterval? Yes, AT Present S_(n)

1st Indicated S_(n)

2nd Indicated Pacing Interval Pacing Interval (i.e., VRR) (e.g., Sensor)No, AT not Present S_(n)

2nd Indicated S_(n)

2nd Indicated Pacing Interval Pacing Interval (e.g., Sensor) (e.g.,Sensor)

[0071] In this embodiment, if an atrial tachyarrhythmia is present andthe first indicated pacing interval is shorter than the second indicatedpacing interval, then selection module 915 selects the first indicatedpacing interval, which is based on the VRR techniques described above,as the selected indicated pacing interval S_(n). Otherwise, selectionmodule 915 selects the second indicated pacing interval, which in oneembodiment is based on the sensor indications, as the selected indicatedpacing interval S_(n). As discussed above, if no ventricular beat issensed during the selected indicated pacing interval S_(n), which ismeasured as the time from the occurrence of the ventricular beatconcluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the selected indicatedpacing interval S_(n).

[0072] Stated differently, for this embodiment, the ventricle is pacedat the VRR indicated rate only if an atrial tachyarrhythmia, such asatrial fibrillation, is present and the VRR indicated rate exceeds thesensor indicated rate. Otherwise the ventricle is paced at the sensorindicated rate. If, for example, the patient is resting, such that thesensor indicated rate is lower than the patient's intrinsic rate, and noatrial tachyarrhythmia is present, then the device will sense theintrinsic rate or will deliver ventricular paces at the lower ratelimit. But if, for example, the patient is resting, but with an atrialtachyarrhythmia that induces irregular ventricular contractions, thenpacing pulses generally will be delivered at the VRR indicated rate. Inanother example, if the patient is active, such that the sensorindicated rate is higher than the VRR indicated rate, then pacing pulsesgenerally will be delivered at the sensor indicated rate, whether or notatrial tachyarrhythmia is present. As an alternative to the selectiondescribed with respect to Table 1, selection module 915 provides a fixedor variable weighting or blending of both the sensor-indicated rate andVRR indicated rate, such that pacing pulses are delivered based on theblended rate.

[0073] The second indicated pacing interval need not be based on sensorindications. In one embodiment, for example, the second indicated pacinginterval tracks the sensed atrial heart rate when no atrialtachyarrhythmia is present. In this embodiment, selection module 915performs a mode-switching function in which the first indicated pacinginterval is used whenever atrial tachyarrhythmia is present and thesecond indicated pacing interval (e.g., atrial-tracking) is used when noatrial tachyarrhythmia is present.

[0074] In another embodiment, heart rate/interval is used as a triggerturn on/off use of the first indicated pacing interval (e.g., the VRRindicated pacing interval). In one example, pacing therapy is based onthe first indicated pacing interval if the first indicated pacinginterval is longer than a first predetermined value, and pacing therapyis substantially independent of the first indicated pacing interval ifthe first indicated pacing interval is shorter than the firstpredetermined value. In this example, the VRR indicated pacing intervalis used at low heart rates, but not at fast heart rates.

Filter Rate Behavior Example 1

[0075]FIG. 11 is a graph illustrating generally, by way of example, butnot by way of limitation, one embodiment of a VRR indicated rate forsuccessive ventricular heart beats for one mode of operating filter 515.As discussed above, the VRR indicated rate is simply the frequency,between ventricular heart beats, associated with the first indicatedpacing interval. Stated differently, the VRR indicated rate is theinverse of the duration of the first indicated pacing interval. Ifpacing is based solely on the VRR indicated rate, pacing control module505 directs ventricular therapy circuit 320 to issue a pacing pulseafter the time since the last ventricular beat equals or exceeds thefirst indicated pacing interval. However, as described above, in certainembodiments, pacing control module 505 directs ventricular therapycircuit 320 to issue a pacing pulse based on factors other than the VRRindicated rate such as for, example, based on the sensor indicated rate.

[0076] In the example illustrated in FIG. 11, a first sensed intrinsicventricular beat, indicated by an “S” was detected just beforeexpiration of the first indicated pacing interval (“VRR indicated pacinginterval”) T₀, as computed based on a previous ventricular beat. In oneembodiment, the new VRR indicated pacing interval T₁ is computed basedon the duration of most recent V-V interval VV₁ and a previous value ofthe VRR indicated pacing interval T₀, as discussed above. In thisexample, the new VRR indicated pacing interval T₁ corresponds to a lowerrate limit (LRL) time interval. In one embodiment, the allowable rangeof the VRR indicated pacing interval is limited so that the VRRindicated pacing interval does not exceed the duration of the LRL timeinterval, and so that the VRR indicated pacing interval is not shorterthan the duration of an upper rate limit (URL) time interval.

[0077] The second ventricular beat is also sensed, just beforeexpiration of the VRR indicated pacing interval T₁. In one embodiment,the new VRR indicated pacing interval T₂ is computed based on theduration of most recent V-V interval VV₂ and a previous value of the VRRindicated pacing interval, T₁, as discussed above. The first and secondventricular beats represent a stable intrinsic rhythm, for which nopacing is delivered because the VRR indicated pacing interval is at alower rate than the sensed intrinsic ventricular beats.

[0078] The third, fourth, and fifth ventricular beats represent theonset of atrial fibrillation, resulting in erratic ventricular rates.The third ventricular beat is sensed well before expiration of the VRRindicated pacing interval T₂, such that no pacing pulse is issued. Forthe sensed third ventricular beat, filter 515 computes the new VRRindicated pacing interval T₃ as being shorter in duration relative tothe previous VRR indicated pacing interval T₂.

[0079] The fourth ventricular beat is similarly sensed well beforeexpiration of the VRR indicated pacing interval T₃, such that no pacingpulse is issued. For the sensed fourth ventricular beat, filter 515computes the new VRR indicated pacing interval T₄ as being shorter induration relative to the previous VRR indicated pacing interval T₃.

[0080] The fifth ventricular beat is sensed before expiration of the VRRindicated pacing interval T₄, such that no pacing pulse is issued. Forthe sensed fifth ventricular beat, filter 515 computes the new VRRindicated pacing interval T₅ as being shorter in duration relative tothe previous VRR indicated pacing interval T₄.

[0081] The sixth, seventh, and eighth ventricular beats indicateregularization of the ventricular rate using the pacing techniquesdescribed above. No ventricular beat is sensed during the VRR indicatedpacing interval T₅, so a pacing pulse is issued to evoke the sixthventricular beat. A new VRR indicated pacing interval T₆ is computed asbeing increased in duration relative to the previous VRR indicatedpacing interval T₅, lowering the VRR indicated rate. Similarly, noventricular beat is sensed during the VRR indicated pacing interval.

[0082] The ninth ventricular beat represents another erratic ventricularbeat resulting from the atrial fibrillation episode. The ninthventricular beat is sensed before expiration of the VRR indicated pacinginterval T₈. As a result, a shorter new VRR indicated pacing interval T₉is computed.

[0083] The tenth and eleventh ventricular beats illustrate furtherregularization of the ventricular rate using the pacing techniquesdescribed above. No ventricular beat is sensed during the VRR indicatedpacing interval T₉, so a pacing pulse is issued to evoke the tenthventricular beat. A new VRR indicated pacing interval T₁₀ is computed asbeing increased in duration relative to the previous VRR indicatedpacing interval T₉, lowering the VRR indicated rate. Similarly, noventricular beat is sensed during the VRR indicated pacing interval T₁₀,so a pacing pulse is issued to evoke the tenth ventricular beat. A newVRR indicated pacing interval T₁₁ is compute as being increased induration relative to the previous VRR indicated pacing interval T₁₀,lowering the VRR indicated rate.

[0084] The twelfth, thirteenth, fourteenth, and fifteenth ventricularbeats illustrate resumption of a stable intrinsic rhythm aftertermination of the atrial fibrillation episode. For such a stable rate,the VRR indicated rate proceeds asymptotically toward a “floor value”that tracks, but remains below, the intrinsic rate. This allows theintrinsic heart signals to control heart rate when such intrinsic heartsignals provide a stable rhythm. As a result, when the patient'sintrinsic rate is constant, paces will be withheld, allowing thepatient's intrinsic heart rhythm to continue. If the patient's heartrate includes some variability, and the VRR indicated floor value isclose to the mean intrinsic heart rate, then occasional paced beats willoccur. Such pace beats will gradually lengthen the VRR indicated pacinginterval, thereby allowing subsequent intrinsic behavior when thepatient's heart rate becomes substantially constant.

[0085] The intrinsic coefficient a of filter 515 controls the “attackslope” of the VRR indicated heart rate as the VRR indicated heart rateincreases because of sensed intrinsic beats. The paced coefficient b offilter 515 controls the “decay slope” of the VRR indicated heart rate asthe VRR indicated heart rate decreases during periods of paced beats. Inone embodiment, in which a>1.0 and b>1.0, decreasing the value of atoward 1.0 increases the attack slope such that the VRR indicated rateincreases faster in response to sensed intrinsic beats, while decreasingthe value of b toward 1.0 decreases the decay slope such that the VRRindicated rate decreases more slowly during periods of paced beats.Conversely, for a>1.0 and b>1.0, increasing the value of a from 1.0decreases the attack slope such that the VRR indicated rate increasesmore slowly in response to sensed intrinsic beats, while increasing thevalue of b from 1.0 increases the decay slope such that theVRR-indicated rate decreases more quickly during periods of paced beats.

[0086] In one embodiment, for a>1.0 and b>1.0, decreasing both a and btoward 1.0 increases VRR indicated rate during periods of sensedintrinsic activity so that the VRR indicated rate is closer to the meanintrinsic rate. Because the VRR indicated rate is closer to the meanintrinsic rate, variability in the intrinsic heart rate is more likelyto trigger paces at the VRR indicated rate. On the other hand, for a>1.0and b >1.0, increasing both a and b from 1.0 decreases the VRR indicatedrate during periods of sensed intrinsic activity so that the VRRindicated rate is farther beneath the mean intrinsic rate. Because theVRR indicated rate is farther beneath the mean intrinsic rate, the samevariability in the intrinsic heart rate becomes less likely to triggerpaces at the VRR indicated rate.

[0087] In one embodiment, these coefficients are programmable by theuser, such as by using remote programmer 125. In another embodiment, theuser selects a desired performance parameter (e.g., desired degree ofrate regularization, desired attack slope, desired decay slope, etc.)from a corresponding range of possible values, and device 105automatically selects the appropriate combination of coefficients offilter 515 to provide a filter setting that corresponds to the selecteduser-programmed performance parameter, as illustrated generally by Table2. Other levels of programmability or different combinations ofcoefficients may also be used. TABLE 2 Example of Automatic Selection ofAspects of Filter Setting Based on a User-Programmable PerformanceParameter. User-Programmable Performance Parameter Intrinsic Coefficienta Paced Coefficient b 1 (Less Rate 2.0 3.0 Regularization) 2 1.8 2.6 31.6 2.2 4 1.4 1.8 5 1.2 1.4 6 (More Rate 1.0 1.0 Regularization)

Filter Rate Behavior Example 2

[0088]FIG. 12 is a graph illustrating generally, by way of example, butnot by way of limitation, one embodiment of selecting between more thanone indicated pacing interval. FIG. 12 is similar to FIG. 11 in somerespects, but FIG. 12 includes a second indicated pacing interval. Inone embodiment, the first indicated pacing interval is the VRR indicatedpacing interval, described above, and the second indicated pacinginterval is a sensor indicated pacing interval, from an accelerometer,minute ventilation, or other indication of the patient's physiologicalneed for increased cardiac output.

[0089] In one embodiment, a selected indicated pacing interval is basedon the shorter of the first and second indicated pacing intervals.Stated differently, device 105 provides pacing pulses at the higherindicated pacing rate. In the example illustrated in FIG. 12, first andsecond beats and the twelfth through fifteenth beats are paced at thesensor indicated rate, because it is higher than the VRR indicated rateand the intrinsic rate. The third, fourth, fifth, and ninth beats aresensed intrinsic beats that are sensed during the shorter of either ofthe VRR and sensor indicated pacing intervals. The sixth through eighthbeats and tenth and eleventh beats are paced at the VRR indicated rate,because it is higher than the sensor indicated rate. Also, for thesebeats, no intrinsic beats are sensed during the VRR indicated intervals.In one embodiment, the above-described equations for filter 515 operateto increase the VRR indicated rate toward the sensor-indicated rate whenthe sensor indicated rate is greater than the VRR indicated rate, asillustrated by first through third and twelfth through fifteenth beatsin FIG. 12. In an alternate embodiment, however,T_(n)=b·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by a VRRindicated paced beat, and T_(n)=T_(n-1) if VV_(n) is concluded by asensor indicated paced beat, thereby leaving the VRR indicated rateunchanged for sensor indicated paced beats.

[0090] In this embodiment, the ranges of both the sensor indicated rateand the VRR indicated rate are limited so that they do not extend torates higher than the URL or to rates lower than the LRL. In oneembodiment, the LRL and the URL are programmable by the user, such as byusing remote programmer 125.

[0091] In a further embodiment, the selected indicated pacing intervalis based on the shorter of the first and second indicated pacingintervals only if an atrial tachyarrhythmia, such as atrialfibrillation, is present. Otherwise, the second indicated pacinginterval is used, as described above.

Filter Rate Behavior Example 3

[0092]FIG. 13 is a graph illustrating generally, by way of example, butnot by way of limitation, another illustrative example of heart rate vs.time according to a spreadsheet simulation of the behavior of theabove-described VRR algorithm. In FIG. 13, the VRR algorithm is turnedoff until time 130. Stable intrinsic lower rate behavior is modeled fortimes between 0 and 10 seconds. Erratic intrinsic ventricular rates,such as would result from atrial tachyarrhythmias including atrialfibrillation, are modeled during times between 10 seconds and 130seconds. At time 130 seconds, the VRR algorithm is turned on. While someerratic intrinsic beats are subsequently observed, the VRR algorithmprovides pacing that is expected to substantially stabilize the heartrate, as illustrated in FIG. 13. The VRR indicated pacing rate graduallydecreases until intrinsic beats are sensed, which results in a slightincrease in the VRR indicated pacing rate. Thus, the VRR algorithmfavors the patient's intrinsic heart rate when it is stable, and pacesat the VRR indicated heart rate when the patient's intrinsic heart rateis unstable. It is noted that FIG. 13 does not represent clinical data,but rather provides a simulation model that illustrates one example ofhow the VRR algorithm is expected to operate.

Filter Example 4

[0093] In one embodiment, filter 515 includes variable coefficients suchas, for example, coefficients that are a function of heart rate (or itscorresponding time interval). In one example, operation of the filter515 is described by T_(n)=a·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) isconcluded by an intrinsic beat, otherwise is described byT_(n)=b·w·VV_(n)+(1−w)·T_(n-1), if VV_(n) is concluded by a paced beat,where at least one of a and b are linear, piecewise linear, or nonlinearfunctions of one or more previous V-V intervals such as, for example,the most recent V-V interval, VV_(n).

[0094]FIG. 14 is a graph illustrating generally, by way of example, butnot by way of limitation, one embodiment of using at least one ofcoefficients a and b as a function of one or more previous V-V intervalssuch as, for example, the most recent V-V interval VV_(n). In one suchexample, a is less than 1.0 when VV_(n) is at or near the lower ratelimit (e.g., 1000 millisecond interval or 60 beats/minute), and a isgreater than 1.0 when VV_(n) is at or near the upper rate limit (e.g.,500 millisecond interval or 120 beats/minute). For a constant b, using asmaller value of a at lower rates will increase the pacing rate morequickly for sensed events; using a larger value of a at higher ratesincreases the pacing rate more slowly for sensed events. In anotherexample, b is close to 1.0 when VV_(n) is at or near the lower ratelimit, and b is greater than 1.0 when VV, is at or near the upper ratelimit. For a constant a, using a smaller value of b at lower rates willdecrease the pacing rate more slowly for paced events; using a largervalue of b at higher rates decreases the pacing rate more quickly forpaced events.

Conclusion

[0095] It is to be understood that the above description is intended tobe illustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A method of delivering biventricular coordinationtherapy, comprising: initiating an atrioventricular pacing delay for afirst cardiac cycle relative to an atrial event paced or sensed usingone or more atrial electrodes; decreasing the atrioventricular pacingdelay for a second cardiac cycle, occurring after the first cardiaccycle, if an intrinsic ventricular depolarization is sensed beforeexpiration of the atrioventricular pacing delay for the first cardiaccycle; and delivering the biventricular coordination therapy during thesecond cardiac cycle, using one or more right ventricular electrodes andone or more left ventricular electrodes separate from the one or moreright ventricular electrodes, using the decreased atrioventricularpacing delay.
 2. The method of claim 1, wherein initiating theatrioventricular pacing delay relative to the atrial event comprisesinitiating the atrioventricular pacing delay relative to an intrinsicatrial depolarization sensed using the one or more atrial electrodes. 3.The method of claim 1, wherein initiating the atrioventricular pacingdelay relative to the atrial event comprises initiating theatrioventricular pacing delay relative to delivery of an atrial pacingpulse using the one or more atrial electrodes.
 4. The method of claim 1,wherein delivering the biventricular coordination therapy comprises,using the decreased atrioventricular pacing delay, pacing the leftventricle using the one or more left ventricular electrodes, and pacingthe right ventricle using the one or more right ventricular electrodes.5. The method of claim 4, wherein pacing the left ventricle and pacingthe right ventricle comprises pacing at least a first one of the leftand the right ventricle after the decreased atrioventricular pacingdelay.
 6. The method of claim 4, further comprising sensing for theintrinsic ventricular depolarization using the one or more rightventricular electrodes.
 7. The method of claim 6, wherein pacing theright ventricle and sensing for the intrinsic ventricular depolarizationusing the one or more right ventricular electrodes comprises pacing andsensing using a common electrode of the one or more right ventricularelectrodes.
 8. The method of claim 6, wherein pacing the right ventricleand sensing for the intrinsic ventricular depolarization using the oneor more right ventricular electrodes comprises pacing and sensing usingdifferent electrodes of the one or more right ventricular electrodes. 9.The method of claim 4, further comprising sensing for the intrinsicventricular depolarization using the one or more left ventricularelectrodes.
 10. The method of claim 9, wherein pacing the left ventricleand sensing for the intrinsic ventricular depolarization using the oneor more left ventricular electrodes comprises pacing and sensing using acommon electrode of the one or more left ventricular electrodes.
 11. Themethod of claim 9, wherein pacing the left ventricle and sensing for theintrinsic ventricular depolarization using the one or more leftventricular electrodes comprises pacing and sensing using a differentelectrode of the one or more left ventricular electrodes.
 12. The methodof claim 1, wherein delivering the biventricular coordination therapycomprises: sensing, during the second cardiac cycle, for an intrinsicdepolarization of the right ventricle using the one or more rightventricular electrodes; and pacing the left ventricle, during the secondcardiac cycle, using the one or more left ventricular electrodes usingan interventricular pacing delay.
 13. The method of claim 1, whereindelivering the biventricular coordination therapy comprises: sensing,during the second cardiac cycle, for an intrinsic depolarization of theleft ventricle using the one or more left ventricular electrodes; andpacing the right ventricle, during the second cardiac cycle, using theone or more right ventricular electrodes using an interventricularpacing delay.
 14. The method of claim 1, wherein delivering thebiventricular coordination therapy comprises: sensing, during the secondcardiac cycle, for an intrinsic depolarization of one of the rightventricle or left ventricle using the respective one or more right orleft ventricular electrodes; and pacing both the left ventricle andright ventricle, during the second cardiac cycle, using the one or moreleft ventricular electrodes and the one or more right ventricularelectrodes upon sensing the intrinsic depolarization.
 15. The method ofclaim 1, wherein delivering the biventricular coordination therapycomprises delivering a first pacing pulse to the right ventricle usingthe one or more right ventricular electrodes and a second pacing pulseto the left ventricle using the one or more left ventricular electrodes,the first and the second pacing pulses delivered substantiallysimultaneously.
 16. The method of claim 1, wherein the intrinsicventricular depolarization sensed during the atrioventricular delay forthe first cardiac cycle comprises a right ventricular intrinsicdepolarization sensed using the one or more right ventricularelectrodes.
 17. The method of claim 1, wherein the intrinsic ventriculardepolarization sensed during the atrioventricular delay for the firstcardiac cycle comprises an left ventricular intrinsic depolarizationsensed using the one or more left ventricular electrodes.
 18. The methodof claim 1, further comprising: increasing the atrioventricular pacingdelay for a third cardiac cycle, after the second cardiac cycle, if anintrinsic ventricular depolarization is not sensed during anatrioventricular pacing delay for a cardiac cycle immediately precedingthe third cardiac cycle; and delivering a biventricular coordinationtherapy during the third cardiac cycle using the increasedatrioveritricular pacing delay.
 19. The method of claim 18, whereinincreasing the atrioventricular pacing delay for the third cardiac cycleif the intrinsic depolarization is not sensed during theatrioventricular pacing delay for the cardiac cycle immediatelypreceding the third cardiac cycle comprises increasing theatrioventricular pacing delay for the third cardiac cycle if a pace isdelivered to the right ventricle or the left ventricle using theatrioventricular pacing delay for the cardiac cycle immediatelypreceding the third cardiac cycle.
 20. The method of claim 1, whereindecreasing the atrioventricular pacing delay for the second cardiaccycle comprises decreasing the atrioventricular pacing delay based on anintrinsic atrioventricular interval.
 21. The method of claim 1, furthercomprising limiting the atrioventricular pacing delay during atachyarrhythmia episode.
 22. The method of claim 1, further comprisinglimiting the atrioventricular pacing delay during an atrialtachyarrhythmia episode.
 23. The method of claim 1, further comprisingsensing hemodynamic need of a patient and decreasing theatrioventricular delay for the second cardiac cycle based on the sensedhemodynamic need of the patient.
 24. The method of claim 1, furthercomprising sensing for the intrinsic ventricular depolarization using atleast one of 1) the one or more right ventricular electrodes or 2) theone or more left ventricular electrodes.
 25. The method of claim 1,wherein the second cardiac cycle immediately follows the first cardiaccycle.
 26. The method of claim 1, wherein delivering the biventricularcoordination therapy includes at least one of pacing a right ventricleand sensing for a right ventricular intrinsic depolarization and atleast one of pacing a left ventricle and sensing for a left ventriculardepolarization.
 27. The method of claim 1, wherein the decreasing theatrioventricular pacing delay comprises decreasing the atrioventricularpacing delay within a predetermined limit.
 28. The method of claim 27,wherein the predetermined limit comprises a minimum atrioventricularpacing delay.
 29. A method of delivering biventricular coordinationtherapy, comprising: initiating an atrioventricular pacing delay for afirst cardiac cycle relative to an atrial event paced or sensed usingone or more atrial electrodes; modifying the atrioventricular pacingdelay for a second cardiac cycle, occurring after the first cardiaccycle, if an intrinsic ventricular depolarization is sensed beforeexpiration of the atrioventricular pacing delay for the first cardiaccycle; and delivering the biventricular coordination therapy during thesecond cardiac cycle using one or more right ventricular electrodes andone or more left ventricular electrodes separate from the one or moreright ventricular electrodes using the decreased atrioventricular pacingdelay.
 30. A method of delivering biventricular coordination therapy,comprising: sensing an atrium using one or more atrial electrodes;sensing and pacing a right ventricle using one or more right ventricularelectrodes; pacing a left ventricle using one or more left ventricularelectrodes; initiating an atrioventricular pacing delay for a firstcardiac cycle relative to an atrial event that is paced or sensed usingthe one or more atrial electrodes; sensing an intrinsic ventriculardepolarization using the one or more right ventricular electrodes beforeexpiration of the atrioventricular pacing delay; decreasing theatrioventricular pacing delay for a second cardiac cycle, occurringafter the first cardiac cycle, after sensing the intrinsic ventriculardepolarization using the one or more right ventricular electrodes; andin the second cardiac cycle, pacing the right ventricle using the one ormore right ventricular electrodes and pacing the left ventricle usingthe one or more left ventricular electrodes using the decreasedatrioventricular pacing delay.
 31. The method of claim 30, furthercomprising, in a third cardiac cycle, occurring after the second cardiaccycle, increasing the atrioventricular pacing delay based on thedelivery of a ventricular pace.
 32. The method of claim 31, wherein thethird cardiac cycle immediately follows the second cardiac cycle.
 33. Abiventricular coordination therapy device, comprising: one or moreatrial electrodes for electrically coupling to an atrium; one or moreright ventricular electrodes for electrically coupling to a rightventricle; one or more left ventricular electrodes for electricallycoupling to a left ventricle; and a pulse generator coupled to the oneor more atrial electrodes, the one or more right ventricular electrodes,and the one or more left ventricular electrodes, the pulse generatorconfigured to initiate an atrioventricular pacing delay for a firstcardiac cycle relative to an atrial event paced or sensed using the oneor more atrial electrodes, decrease the atrioventricular pacing delayfor a second cardiac cycle, occurring after the first cardiac cycle, ifan intrinsic ventricular depolarization is sensed during theatrioventricular pacing delay for the first cardiac cycle, and deliverthe biventricular coordination therapy during the second cardiac cycleusing the one or more right ventricular electrodes and the one or moreleft ventricular electrodes using the decreased atrioventricular pacingdelay.
 34. The device of claim 33, wherein the pulse generator isconfigured to initiate the atrioventricular pacing delay relative to anintrinsic atrial depolarization.
 35. The device of claim 33, wherein thepulse generator is configured to initiate the atrioventricular pacingdelay relative to an atrial pacing pulse.
 36. The device of claim 33,wherein the pulse generator is configured to, using the decreasedatrioventricular pacing delay, deliver a right ventricular pacing pulseto the right ventricle using the one or more right ventricularelectrodes and deliver a left ventricular pacing pulse to the leftventricle using the one or more left ventricular electrodes.
 37. Thedevice of claim 36, wherein the pulse generator is configured to deliverat least a first one of the right ventricular pacing pulse and the leftventricular pacing pulse after the decreased atrioventricular delay. 38.The device of claim 36, wherein the pulse generator is configured tosense for the intrinsic ventricular depolarization using the one or moreright ventricular electrodes.
 39. The device of claim 38, wherein thepulse generator is configured to use a common electrode of the one ormore right ventricular electrodes to sense for the intrinsic ventriculardepolarization and deliver the right ventricular pacing pulse to theright ventricle.
 40. The device of claim 38, wherein the pulse generatoris configured to use one electrode of the one or more right ventricularelectrodes to sense for the intrinsic ventricular depolarization andanother of the one or more right ventricular electrodes to deliver theright ventricular pacing pulse to the right ventricle.
 41. The device ofclaim 36, wherein the pulse generator is configured to sense for theintrinsic ventricular depolarization using the one or more leftventricular electrodes.
 42. The device of claim 41, wherein the pulsegenerator is configured to use a common electrode of the one or moreleft ventricular electrodes to sense for the intrinsic ventriculardepolarization and deliver the left ventricular pacing pulse to the leftventricle.
 43. The device of claim 41, wherein the pulse generator isconfigured to use one electrode of the one or more left ventricularelectrodes to sense for the intrinsic ventricular depolarization andanother of the one or more left ventricular electrodes to deliver theleft ventricular pacing pulse to the left ventricle.
 44. The device ofclaim 33, wherein the pulse generator is configured to sense, during thesecond cardiac cycle, for an intrinsic depolarization of the rightventricle using the one or more right ventricular electrodes and todeliver, during the second cardiac cycle, a pacing pulse to the leftventricle using the one or more left ventricular electrodes using aninterventricular pacing delay.
 45. The device of claim 33, wherein thepulse generator is configured to sense, during the second cardiac cycle,for an intrinsic depolarization of the left ventricle using the one ormore left ventricular electrodes and to deliver, during the secondcardiac cycle, a pacing pulse to the right ventricle using the one ormore right ventricular electrodes using an interventricular pacingdelay.
 46. The device of claim 33, wherein the pulse generator isconfigured to sense, during the second cardiac cycle, for an intrinsicdepolarization of one of the right ventricle and the left ventricle and,upon sensing the intrinsic depolarization, pace the left ventricle usingthe one or more left ventricular electrodes during the second cardiaccycle and pace the right ventricle using the one or more rightventricular electrodes during the second cardiac cycle.
 47. The deviceof claim 33, wherein the pulse generator is configured to deliver afirst pacing pulse to the right ventricle using the one or more rightventricular electrodes and deliver a second pacing pulse to the leftventricle using the one or more left ventricular electrodes, the firstand the second pacing pulses delivered substantially simultaneously. 48.The device of claim 33, wherein the pulse generator is configured usethe one or more right ventricular electrodes to sense a rightventricular intrinsic depolarization during the atrioventricular delayfor the first cardiac cycle.
 49. The device of claim 33, wherein thepulse generator is configured use the one or more right left electrodesto sense a left ventricular intrinsic depolarization during theatrioventricular delay for the first cardiac cycle.
 50. The device ofclaim 33, wherein the pulse generator is configured to increase theatrioventricular pacing delay for a third cardiac cycle, occurring afterthe second cardiac cycle, if an intrinsic ventricular depolarization isnot sensed during an atrioventricular pacing delay for a cardiac cycleimmediately preceding the third cardiac cycle and deliver biventricularcoordination therapy during the third cardiac cycle using the increasedatrioventricular delay.
 51. The device of claim 50, wherein the pulsegenerator is configured to increase the atrioventricular pacing delayfor the third cardiac cycle if a pace is delivered to the rightventricle or the left ventricle using the atrioventricular pacing delayfor the cardiac cycle immediately preceding the third cardiac cycle. 52.The device of claim 33, wherein the pulse generator is configured todecrease the atrioventricular pacing delay for the second cardiac cyclebased on an intrinsic atrioventricular interval.
 53. The device of claim33, wherein the pulse generator is configured to limit theatrioventricular pacing delay during a tachyarrhythmia episode.
 54. Thedevice of claim 33, wherein the pulse generator is configured to limitthe atrioventricular pacing delay during an atrial tachyarrhythmiaepisode.
 55. The device of claim 33, further comprising a sensing systemconfigured to sense hemodynamic need of a patient, wherein the pulsegenerator is configured to adjust the atrioventricular pacing delaybased on the sensed hemodynamic need of the patient.
 56. The device ofclaim 55, wherein the sensing system comprises an activity sensor. 57.The device of claim 55, wherein the sensing system comprises arespiration sensor.
 58. The device of claim 33, wherein the pulsegenerator is configured to sense for the intrinsic ventriculardepolarization using at least one of 1) the one or more rightventricular electrodes or 2) the one or more left ventricularelectrodes.
 59. The device of claim 33, wherein the second cardiac cycleimmediately follows the first cardiac cycle.
 60. The device of claim 33,wherein the biventricular coordination therapy comprises at least one ofpacing a right ventricle and sensing for a right ventricular intrinsicdepolarization and at least one of pacing a left ventricle and sensingfor a left ventricular depolarization.
 61. The device of claim 33,wherein the pulse generator is configured to decrease theatrioventricular pacing delay within a predetermined limit.
 62. Thedevice of claim 61, wherein the predetermined limit comprises a minimumatrioventricular pacing delay.
 63. A biventricular coordination therapydevice, comprising: one or more atrial electrodes configured toimplement at least one of sensing and pacing an atrium; one or moreright ventricular electrodes configured to implement at least one ofsensing and pacing a right ventricle; one or more left ventricularelectrodes configured to implement at least one of sensing and pacing aleft ventricle; and a pulse generator coupled to the one or more atrialelectrodes, the one or more right ventricular electrodes, and the one ormore left ventricular electrodes, the pulse generator configured toinitiate an atrioventricular pacing delay for a first cardiac cyclerelative to an atrial event paced or sensed using the one or more atrialelectrodes, modify the atrioventricular pacing delay for a secondcardiac cycle, occurring after the first cardiac cycle, if an intrinsicventricular depolarization is sensed before expiration of theatrioventricular pacing delay for the first cardiac cycle, and deliver abiventricular coordination therapy during the second cardiac cycle usingthe one or more right ventricular electrodes and the one or more leftventricular electrodes separate from the one or more right ventricularelectrodes using the modified atrioventricular pacing delay.
 64. Abiventricular coordination therapy device, comprising: one or moreatrial electrodes configured to sense an atrium; one or more rightventricular electrodes configured to sense and pace a right ventricle;one or more left ventricular electrodes configured to pace a leftventricle; and a pulse generator coupled to the one or more atrialelectrodes, the one or more right ventricular electrodes, and the one ormore left ventricular electrodes, the pulse generator configured toinitiate an atrioventricular pacing delay for a first cardiac cyclerelative to an atrial event paced or sensed using the one or more atrialelectrodes, decrease the atrioventricular pacing delay for a secondcardiac cycle, occurring after the first cardiac cycle, after sensing anintrinsic ventricular depolarization before expiration of theatrioventricular pacing delay for the first cardiac cycle, and, duringthe second cardiac cycle, pace the right ventricle using the one or moreright ventricular electrodes and pace the left ventricle using the oneor more left ventricular electrodes using the decreased atrioventricularpacing delay.
 65. The device of claim 64, wherein the pulse generator isconfigured to increase the atrioventricular pacing delay for a thirdcardiac cycle, occurring after the second cardiac cycle, based on thedelivery of a ventricular pace during the second cardiac cycle.
 66. Thedevice of claim 65, wherein the third cardiac cycle immediately followsthe second cardiac cycle.
 67. A system for delivering biventricularcoordination therapy to a patient, comprising: means for initiating anatrioventricular pacing delay for a first cardiac cycle relative to anatrial event paced or sensed using one or more atrial electrodes; meansfor decreasing the atrioventricular pacing delay for a second cardiaccycle, occurring after the first cardiac cycle, if an intrinsicventricular depolarization is sensed before expiration of theatrioventricular pacing delay for the first cardiac cycle; and means fordelivering the biventricular coordination therapy during the secondcardiac cycle, using one or more right ventricular electrodes and one ormore left ventricular electrodes separate from the one or more rightventricular electrodes, using the decreased atrioventricular pacingdelay.
 68. The system of claim 67, further comprising: means forsensing, during the second cardiac cycle, for an intrinsicdepolarization of the right ventricle using the one or more rightventricular electrodes; and means for pacing the left ventricle, duringthe second cardiac cycle, using the one or more left ventricularelectrodes using an interventricular pacing delay.
 69. The system ofclaim 67, further comprising: means for sensing, during the secondcardiac cycle, for an intrinsic depolarization of the left ventricleusing the one or more left ventricular electrodes; and means for pacingthe right ventricle, during the second cardiac cycle, using the one ormore right ventricular electrodes using an interventricular pacingdelay.
 70. The system of claim 67, further comprising: means forsensing, during the second cardiac cycle, for an intrinsicdepolarization of one of the right ventricle or left ventricle using therespective one or more right or left ventricular electrodes; and meansfor pacing both the left ventricle and right ventricle, during thesecond cardiac cycle, using the one or more left ventricular electrodesand the one or more right ventricular electrodes upon sensing theintrinsic depolarization.
 71. The system of claim 67, furthercomprising: means for increasing the atrioventricular pacing delay for athird cardiac cycle, after the second cardiac cycle, if an intrinsicventricular depolarization is not sensed during an atrioventricularpacing delay for a cardiac cycle immediately preceding the third cardiaccycle; and means for delivering a biventricular coordination therapyduring the third cardiac cycle using the increased atrioventricularpacing delay.
 72. The system of claim 67, further comprising means fordecreasing the atrioventricular pacing delay within a predeterminedlimit.
 73. The system of claim 67, further comprising means for sensinghemodynamic need of the patient and decreasing the atrioventriculardelay for the second cardiac cycle based on the sensed hemodynamic needof the patient.
 74. A system for delivering biventricular coordinationtherapy, comprising: means for initiating an atrioventricular pacingdelay for a first cardiac cycle relative to an atrial event paced orsensed using one or more atrial electrodes; means for modifying theatrioventricular pacing delay for a second cardiac cycle, occurringafter the first cardiac cycle, if an intrinsic ventriculardepolarization is sensed before expiration of the atrioventricularpacing delay for the first cardiac cycle; and means for delivering thebiventricular coordination therapy during the second cardiac cycle usingone or more right ventricular electrodes and one or more leftventricular electrodes separate from the one or more right ventricularelectrodes using the decreased atrioventricular pacing delay.
 75. Asystem for delivering biventricular coordination therapy, comprising:means for sensing an atrium using one or more atrial electrodes; meansfor sensing and pacing a right ventricle using one or more rightventricular electrodes; means for pacing a left ventricle using one ormore left ventricular electrodes; means for initiating anatrioventricular pacing delay for a first cardiac cycle relative to anatrial event that is paced or sensed using the one or more atrialelectrodes; means for sensing an intrinsic ventricular depolarizationusing the one or more right ventricular electrodes before expiration ofthe atrioventricular pacing delay; means for decreasing theatrioventricular pacing delay for a second cardiac cycle, occurringafter the first cardiac cycle, after sensing the intrinsic ventriculardepolarization using the one or more right ventricular electrodes; andin the second cardiac cycle, means for pacing the right ventricle usingthe one or more right ventricular electrodes and pacing the leftventricle using the one or more left ventricular electrodes using thedecreased atrioventricular pacing delay.
 76. The method of claim 75,further comprising means for increasing, in a third cardiac cycle,occurring after the second cardiac cycle, the atrioventricular pacingdelay based on the delivery of a ventricular pace.
 77. A method foroperating a cardiac pacemaker, comprising: sensing a ventricle so as todetect a ventricular sense upon depolarization occurring in theventricle; pacing a ventricle upon expiration of an atrio-ventricularescape interval initiated by an atrial event and stopped by aventricular sense; measuring an intrinsic atrio-ventricular interval asthe time between an atrial event and a ventricular sense when noventricular pace is delivered; decreasing the atrio-ventricular escapeinterval upon detection of a ventricular sense in a manner dependentupon the measured intrinsic atrio-ventricular interval; and, if nosubsequent ventricular sense is detected, increasing theatrio-ventricular escape interval.
 78. The method of claim 77, furthercomprising delivering paces to both the right and left ventricles uponexpiration of the atrio-ventricular escape interval.
 79. The method ofclaim 77, wherein the atrial event initiating the atrio-ventricularescape interval is an atrial pace.
 80. The method of claim 77, whereinthe atrial event initiating the atrio-ventricular escape interval is anatrial sense.
 81. The method of claim 77, wherein the atrio-ventricularescape interval is decreased by computing a weighted average of themeasured intrinsic atrio-ventricular interval multiplied by a scalingfactor and the atrio-ventricular escape interval.
 82. The method ofclaim 81, wherein the atrio-ventricular escape interval is decreased inaccordance with the measured intrinsic atrio-ventricular interval aftereach ventricular sense.
 83. The method of claim 82, wherein theatrio-ventricular escape interval is increased after each ventricularpace by multiplying the atrio-ventricular escape interval by a specifiedfilter coefficient.